Patients with hypertrophic cardiomyopathy often show substantial increase in left ventricular wall thickness or even de novo development of hypertrophy during childhood. To determine the utility of the electrocardiogram in reflecting such changes in left ventricular mass, serial 12-lead electrocardiograms and two- dimensional echocardiograms were analyzed in 38 children with hypertrophic cardiomyopathy. The results of this study show that children with hypertrophic cardiomyopathy and evolving left ventricular hypertrophy showed little change in the electrocardiogram during the period of observation. However, electrocardiographic abnormalities may be the initial manifestation of hypertrophic cardiomyopathy, appearing even before left ventricular hypertrophy is detectable by echocardiography, thus providing advance evidence of morphologic changes that will subsequently occur.